It is a very telling point and a very true one and it shows quite well the importance of this debate. If evolution is true, then life has no purpose. The lowest and basest instincts are therefore excusable. Eugenics, mercy killing and even abortion become legitimatized. Even mass murder as a form of 'helping' natural selection becomes viable.
One must doubt if any society can survive under such terms. One must doubt if any species can survive which acts in such a way. For the above is a prescription for destruction not for creation. What man needs is hope, not despair in order to better himself and evolution only provides despair.
Arguably, the absence of God makes morality obsolete, although plenty of atheist philosophers argue otherwise, and plenty of non-raping and non-killing atheists show otherwise in practise. Even if one accepts that we need God to define morality absolutely, that does not mean we need an ultra-meddling God who labored over the design of each species, and labored to create a false record of sedimentation, fossilization, etc.
Since you here present utilitarian grounds for traditional morality, you yourself show that even an atheistic utilitarian can accept that your morality is of benefit to society. (And individuals are always apt to see benefit in following the morality of any society with a system of justice, formal or informal.)
BTW, I don't know a lot of people who suffer from "despair," and I doubt very much that despair or major depression or any such thing correlates much one way or the other with belief in evolution.